If the evaporator coil horizontal return line of a container refrigeration system is less than 0.875" (2.21 cm) in diameter (considered small), the thermostatic expansion valve sensing bulb should be attached where on the return line?

Correct Answer: Option C — on the upper surface of the line

For small suction lines under 0.875 inches, oil and liquid refrigerant flowing along the bottom can falsely depress the sensing bulb temperature. Mounting the bulb on the upper surface ensures it accurately senses true suction vapor superheat rather than liquid rivulets.